When dental plaque adhering to teeth is left undisturbed, bacteria in the dental plaque grow, thereby causing tooth decay and periodontal diseases. Proper removal of dental plaque by daily toothbrushing is important for prevention of such tooth decay and periodontal diseases. For effective toothbrushing, manners of moving toothbrushes are devised, and electric toothbrushes are used; however, portions with much dental plaque and portions with little dental plaque are often brushed only for a certain time on an average. Brushing the portions with little dental plaque leads to negative effects of excessive brushing, while the portions with much dental plaque are insufficiently brushed.
Dental plaque can be effectively removed in a short time by concentratedly brushing a portion to which the dental plaque adheres while monitoring the adhering amount of the dental plaque. Therefore, several methods for optically detecting dental plaque have been conventionally proposed. Typical examples thereof include a method utilizing the fact that bacteria included in dental plaque or bacteria in dental caries produce protoporphyrin IX (hereinafter referred to as “PPIX”), which is a fluorescent substance, in the intraoral environment. A fluorescence measuring method has been known in which a tooth is irradiated with excitation light having a certain wavelength, and fluorescence emitted by a fluorescent substance is detected, thereby quantitating the amount of dental plaque or the degree of dental caries.
For example, fluorescence spectra from enamel obtained by irradiating the enamel of a healthy tooth and the enamel of a carious tooth only with excitation light having a wavelength of 406 nm are illustrated in Patent Literature 1. Peaks at wavelengths of 636 nm and 673 nm indicate fluorescent emissions peculiar to carious enamel. Measurement of such fluorescence enables discrimination between the enamel of a healthy tooth and the enamel of a carious tooth. It is found that examination of a fluorescence spectrum is effective for diagnosing dental caries, because the shape of the fluorescence spectrum is obviously changed due to a worsening of dental caries. In addition, Patent Literature 1 describes a method in which the amount S1 of fluorescence having a wavelength band of 636 nm or 673 nm and the amount S2 of fluorescence having a band of 550 nm are measured using a filter, and the ratio S1/S2 thereof is used for quantitative evaluation of dental caries.
The autofluorescence spectrum of a tooth from a tooth surface from which a biological deposit has been removed, and a fluorescence spectrum from a tooth surface covered with a new dental plaque layer are illustrated in Patent Literature 2. The spectra are measured using an excitation light sources at a wavelength of 420 nm or less. In addition, Patent Literature 2 illustrates a magnified view of a region indicating the intensity of autofluorescence at a wavelength longer than 530 nm, and represents that the autofluorescence intensity is decreased with gradually increasing a biological deposition layer. This can be understood to be because excitation light is absorbed or scattered by the dental plaque layer, whereby the intensity of excitation light arriving at the tooth surface is decreased, and thus the autofluorescence from the tooth is reduced.
Patent Literature 2 also describes that the maximum values of strongly emitted light are observed at 530 nm and 630 nm in a fluorescence spectrum obtained from a test tooth surface with a thick layer of a biological deposit considered to be new dental plaque. In the optical system of Patent Literature 2, fluorescence from dental plaque is not directly detected, but a tooth is irradiated with blue light at 480 nm, and the autofluorescence components of the tooth in reflected light are measured using a dichroic mirror. Then, a change in the intensity of autofluorescence from the tooth in movement of a toothbrush along the tooth in toothbrushing is observed to determine whether there is a biological deposition layer or not.
Patent Literature 3 describes a method for preventing the amount of plaque detected by a toothbrush-type plaque detection device from changing even when a distance between a tooth and a toothbrush is changed. Patent Literature 3 primarily describes a method for quantifying secondary fluorescence from a fluorescent agent bound to plaque. This method compensates for an apparent variation in the amount of plaque due to the distance by using a signal from an amplifier, i.e., the total amount of the fluorescence components and reflection components of excitation light. A compensated plaque value is determined using a compensation equation in which a coefficient has been determined by measurement performed in advance.